A process of obtaining ethylene oxide by supplying a feedstock gas containing ethylene and molecular oxygen to a fixed bed comprising a silver catalyst filled in a multi-tube reactor and conducting the reaction at a temperature of 200.degree. to 300.degree. C. has been the only practical industrial process as a process for producing ethylene oxide by oxidation of ethylene with molecular oxygen. It is known that a refractory material is filled in the upstream part of the catalyst-filled bed as a preheating layer to elevate the temperature of the feedstock gas to the prescribed temperature.
The refractory material (packing material) must have the following two requirements: It should not break nor powder when packed or in use and it should not expose the elute sulfur, silicon, alkali metals (e.g., potassium), alkaline earth metals, and heavy metals (e.g., iron or copper) which cause poisoning the catalyst or side reactions other than the desired reaction. The former can be judged by the measurement of the compressive strength, and the latter can be evaluated by the measurement of the solubility in acid. A desirable packing material has higher compressive strength and lower solubility in acid.
Recently, the performance of the silver catalyst has been greatly improved and this means that the improved catalyst is more sensitive to the presence of other elements. Therefore, of the requirements for the packing material in the preheating layer, the latter requirement is particularly severe and high purity alumina which is substantially free of silicon is recently used.
However, a sintered product of high purity alumina is difficult to obtain a high strength and of the requirements for the packing material, the former requirement is not always satisfied. As a result, various problems are encountered that the overall pressure loss in the reactor increases, the local resistance increases and the catalyst does not uniformly contributes to reaction, leading to hot spots. The required strength can, of course, be obtained by sintering at high temperature (e.g., more than 1500.degree. C.), but such is uneconomical. Thus high purity alumina is used without overcoming the above various problems. Further, even if a packing material having a satisfactory strength can be obtained, the problem still remains that elements such as sodium, aluminum, iron, etc. which might poison the silver catalyst tend to be eluted.